In one such closure cap, known from German Patent DE 30 05 419 C2, for a fuel tank, a grip is connected axially and in a manner fixed against relative rotation to one part (sealing part) via a coaxial closing tube, about which closing tube another part (tightening part) is retained axially movably, counter to the action of a spring, in the form of two diametrically opposed locking lugs. A spacer sleeve is disposed between the one part and the other part. The one part carries the sealing ring, which in the locked state of the closure cap rests sealingly on the sealing face of the filler neck.
When this known closure cap, introduced into the filler neck, is manipulated, and upon locking or unlocking rotation relative to the filler neck, the closure cap is pressed together with the sealing ring against the sealing face of the filler neck. Since with the rotation of the grip the sealing ring is also rotated relative to the filler neck, the sealing ring rubs to a greater or lesser extent against the sealing face of the filler neck, depending on the force expended by the user. Although because of the rotary lifting device the known closure cap is designed such that before and after the closing position, an axial play is intrinsically possible between the sealing ring and the sealing face of the filler neck, nevertheless, its existence depends on the use by the user. Moreover, a further increase in the friction of the sealing ring relative to the sealing face of the filler neck can occur if the sealing ring swells from the influence of fuel or hot oil, so that the axial play is reduced still further. Moreover, the spacer sleeve that must additionally be provided is complicated.